1. Field of the Invention
The present invention pertains to the processing of acquired seismic data, and, more particularly, to a method and apparatus for preconditioning seismic data to impart an offset transformation to zero dip that preserves angle of incidence.
2. Description of the Related Art
Seismic surveying traditionally involves imparting acoustic waves from an acoustic source that propagate through subterranean geological formations and are reflected back to seismic sensors. The seismic sensors are arrayed throughout the area being surveyed. The reflected signals are transformed into electrical or optical signals that are then transmitted to a data collection unit. In a land-based survey, the data collection unit is usually housed on a recording truck. In a marine survey, the data collection typically occurs on a survey vessel. The data collection unit either records the signals it receives, analyzes them in real-time, transmits them to a remote location for analysis, or some combination of these things.
The acquired data is then processed in an effort to yield information regarding the location of hydrocarbon or other mineral deposits. More particularly, the data is typically processed to improve the accuracy of the information that can be extracted from it. Data processing techniques, such as “prestack migration,” “poststack migration,” “full prestack waveform inversion,” and “offset-to-angle transformation,” are frequently applied. Still other techniques are known to the art.
However, the complexity of the geological formation's structure can impair the accuracy of these techniques. Many, if not most, processing techniques assume there are no structural dips in the reflectors of the geological formation, i.e., the reflector lies entirely within a horizontal plane relative to the surface. Among the techniques assuming no structural dips are conventional “offset-to-angle conversion” and “full prestack waveform inversion.”
Many of these techniques use the seismic amplitudes of the reflected signals to estimate physical properties of the earth such as P-wave velocity, S-wave velocity, and density. A key factor affecting seismic amplitudes is the angle of incidence of rays as they encounter and are reflected from geologic events. Misrepresentations of incidence angles, such as those that will occur with zero-dip techniques in the presence of structural dip, will contaminate the estimation of the earth properties. Although some techniques may be altered to incorporate structural dip, many can only do so approximately, or not at all.
The art has recognized this problem. See Sherrill, F., et al., “Improving AVO and Reflection Tomography Through Use of Local Dip and Azimuth,” 71st Ann. Intemat. Mtg: Soc. of Expl. Geophys., 273-276 (2001). The article discusses the errors incurred by ignoring structural dip when performing offset-to-angle transformations and reflection tomography. Sherrill et al., started with prestack migrated data (time or depth) to ensure that the data are focused to their proper subsurface locations. A reflector dip field is then derived from the prestack migrated data. From the dip field, ray tracing is used to calculate the angles of incidence from the dipping reflector prior to mapping to the angle domain. This method involves altering zero-dip offset-to-angle transform code to address the effects of structural dip. However, many algorithms may require drastic alteration to incorporate structural dip. In other cases, such corrections may not be theoretically or computationally feasible.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.